Comprehensive Maximum Zs Values for BS 7671 Compliance

Verifying the Earth Fault Loop Impedance (Z_s) is a fundamental statutory requirement during an Electrical Installation Condition Report (EICR) or initial circuit verification. This ensures that the protective device—whether a modern MCB, a legacy MCCB, or a BS 88 cartridge fuse—will disconnect the supply within the required time (e.g., 0.4 s or 5.0 s) under fault conditions.

The 80 % Rule of Thumb Explained

BS 7671 tabulated Z_s limits assume the copper conductors are at their maximum operating temperature (e.g., 70 °C). When testing a circuit on-site, the conductors are usually at ambient room temperature, meaning their resistance is lower. To guarantee the circuit will still trip safely when fully loaded and hot, inspectors apply the 80 % Rule of Thumb (multiplying the maximum Z_s by 0.8). If the measured loop impedance is below this 80 % threshold, the circuit satisfies the requirements of BS 7671 Chapter 41.

BS Fuses & Historic BS 3871 Devices

Finding accurate maximum Z_s limits for obsolete components and traditional fuses can be challenging. Our TMUK Group Ltd master calculator contains data for modern BS EN 60898 devices alongside comprehensive tabulated values for:

• BS 1361 Fuses: Types 1 and 2 cartridge fuses common in domestic service heads.
• BS 88-2 / BS 88-3 Fuses: Industrial cartridge fuses (from 2 A up to 200 A).
• BS 3036 Fuses: Semi-enclosed (rewirable) fuses common in legacy Wylex distribution boards.
• BS 3871 MCBs: Legacy Types 1, 2, 3, and 4 breakers.
• Manufacturer MCCBs: Specific tabulated data for Schneider, Eaton, Dorman Smith, Hager, and Crabtree.

C_min and Voltage Variations

Calculations for generic BS devices in this tool utilise the formula Z_s = (U_o × C_min) / I_a. The C_min factor (0.95) accounts for variations in the voltage supplied by the DNO, effectively reducing the maximum permitted Z_s by 5 % compared to pre-2015 regulations.